The phrase "plant kingdom" refers to a group of living beings that includes all types of plants. It is a classification term used to describe the group of organisms that consists of plants, such as trees, flowers, grass, and moss. Full definition
«The animal [food] world is focused on only a few animals, and we have the whole entire plant kingdom just filled with possibility.» (organicauthority.com)
The whole plant kingdom fits here, along with red and green algae, but not kelps. (sciencenews.org)
It forms the world's smallest and most ecologically diverse plant kingdom. (africanbotanics.com)